Because green hydrogen is expensive, and the more realistic projections estimate it will remain so to at least 2050.

If we use the CRU estimations, which notably are a trade group very much betting on green hydrogen adoption, we should be expecting at best a price of $3 dollars a kg in the distant future, with the current best price being more in the range of $6-$7 dollars a kg. With a 65% effective fuel cell that works out to the hauler getting its electricity at 28 cents a kwh with the possibility of going down to as low as 14 cents a kwh in future.

For reference, the average industrial electricity rate in 2023 was 8 cents per kwh, and Lazard puts the cost of utility scale solar at between 3 and 9 cents a kwh if a mine wants to roll their own. Wholesale Diesel fuel has ranged from about $2 to about $2.8 a gallon so at a reasonable 38% efficiency you get about 14 to 20 cents per kwh.

Not as bad as I expected offhand thanks to the current low cost of industrial green hydrogen, but your still looking at spending a massive amount of money for big platinum catalyst fuel cells and electric powertrains all so that you can spend thirty percent more on fuel than you do currently, as compared to using the grid directly for fifty percent less than you currently spend.

And we haven’t even mentioned that these things are going up and down a steep grade all day, so not only do you want massive batteries anyway so you can recharge on the way down, but you need them because unlike a small FCEV where it’s fuel cell is nowhere near large enough to output full power (and as such is only used to charge the battery) you do actually need to be able to sustain full power for the whole trip up.

TLDR: Industrial electricity is far cheaper than Diesel, let alone the more expensive green hydrogen. Hydrogen may be able to compete on a one or one level with Diesel in the far future, but given the upfront cost of the kit you might as well go all the way so you’re saving money over the long term.